Modern electronic devices like system-in-package SiP devices typically contain several dies which are stacked on top of each other. These dies can be electrically interconnected for example with through-wafer holes such as via interconnects in order to electrically connect the different electronic circuits on the dies. The holes can be filled with conductors such as doped poly-silicon, tungsten and copper. However, the conductor which is filling the via holes must have a high conductivity preferably higher than that for copper due to the skin effect which appears in ultra high frequency applications. Yet, copper is among the highest-conductivity metals and preferred in Si-technology above, e.g. silver. The deposition of copper is for example performed by electroplating which has several disadvantages such as a slow deposition rate, an inhomogeneity of the layer thickness and a non-uniformity of the filling degree. Furthermore, copper is not a favourite design choice in a microelectronic environment as the diffusion barrier layers in the via holes are insufficient such that copper can diffuse into the silicon and can cause an electrical breakdown of p-n junctions. In addition, the wafer stacking requires complicated processing since a multitude of reliable small contacts have to be realized on all dies.
EP 0 526 776 A1 discloses an optoelectric package with a free direct space optical communication between pairs of optical transmitters and receivers located on different substrate surfaces. The transmitters and receivers are aligned such that a light beam from a transmitter follows an optical path to a respective receiver. In the intervening substrates via holes are located to enable the light from the transmitters to pass there through. The transmitter and the receivers are C4-mounted on the respective substrates.